Biologists in Japan studying emus have identified a new developmental reason for their shortened and differently shaped wings.
Emu embryos don’t develop muscles near the tips of their wings. A new study from the Tokyo Institute of Technology shows that without regular muscle movement, the wing bones don’t lengthen as much, and they fuse together in an irregular shape.
Limb reduction and loss has happened several times in vertebrate evolution, notably in snakes, legless lizards and whales. Limb reduction is often associated with animals evolving new habitats and ways of moving, making it an important research area for evolutionary biologists.
As flightless birds with extremely shortened wings, emus (Dromaius novaehollandiae) are an ideal model organism for research into the underlying genetic and developmental mechanisms for limb reduction.
Previous research on emus showed that their embryonic forelimb buds – the structures that form at the beginning of wing development – are relatively smaller and have delayed gene expression compared to flying birds. But this does not explain why individual emus show left-right asymmetry in their wings.
The researchers of the current study used single-cell RNA sequencing to analyse expression patterns of cells within the emu and chicken forelimb bud. In emus, they identified a group of muscle progenitor cells that underwent massive cell death instead of forming muscles near the tip of the wing.
To test whether the loss of muscle movement led to to limb reduction and asymmetry, the researchers used chemicals to experimentally immobilise chicken embryos. They found that immobilised chickens also developed shortened, asymmetrical wings.
The study underscores the critical role of embryonic muscle movement, and the resulting mechanical stress, to bone development. For evolutionary biologists, the findings demonstrate that changes in embryonic muscle movements can drive diversity in animal skeletons.
The research is published in Nature Communications.
